1. Field of the Invention
The present invention relates to a multi-input to single output analog switching device, and more particularly to an analog switching device adapted to prevent at an output stage the appearance of a undesired input signal from circuitry lines connected to a non-selected feature switch when selecting one from many feature switches used in diverse electronic and/or electric products.
2. Description of the Related Art
Recently in such electronic and/or electric products, there has been an attempt to make a wire arrangement so that audio signals from respective products can be transmitted to one output device through feature selection equipment. For example, when the home electronic and/or electric products such as a television set, a video set, an audio set and the like are provided in a home, respective loud speakers must be mounted on the corresponding products to let a user hear each audio source. This has given rise to an inefficiency in view of economics and occupied space. Thus, there is a need for a multi-input to single output analog switching device capable of selecting one audio source among various audio sources from diverse products to transmit the selected source to a single loud speaker system. If a feature switch for the television set is selected in the multi-input to single output analog switching device, the only audio signal transmitted from the television set is produced from the speaker system, while only an audio signal transmitted from the audio set is generated from the speaker system when a feature switch for the audio set is selected.
The conventional multi-input to single output analog switching device is shown in FIG. 1, and its operation will be described with reference to the drawing.
First, suppose that a first switch S1 in a selective switch unit 4 is selected by an output signal of a logic controller 5. When an analog input signal enters from a first input port IN1 of an input stage 1, a first differential amplifier A1 in a differential amplification unit 2 receives the entered analog input signal at its non-inverting terminal and amplifies it, providing the amplified signal to an output stage 8 via a common load stage 3 connected to the output of first differential amplifier A1. However, in the selection of the first switch S1, if another analog input is also applied to a second input port IN2 when the analog input signal was applied to first input port IN1, the another analog input signal is fed to the non-inverting terminal of first differential amplifier A1 through bias resistors RB1 and RB2 of resistor unit 6, and thus appears at the output stage 8 via common load stage 3. This means that, when the analog input signal is not applied to the first input port IN1 but another analog input signal is applied to the second input port IN2, this other analog input signal appears at the output via circuitry lines connected to a non-selected feature switch and a differential amplifier connected to a selected feature switch. As a result, a crosstalk phenomenon may take place.
In FIG. 1, among the unexplained reference numerals, "Vref1" denotes a reference voltage source for differential amplification unit 2, "Ri1" denotes an internal resistor of the reference voltage source, "I" denotes a current source as a bias supply source of the differential amplification unit 2, "RB1 through RB5 denote first through fifth bias resistors, "A1 through A5" denote first through fifth differential amplifiers, "S1 through S5" denote first through fifth switches and "IN1 through IN5" denote first through fifth input terminals.
FIG. 2 shows a detailed circuit diagram of the device of FIG. 1, embodied as an integrated circuit.
As seen in FIG. 2, differential amplification unit 2 comprises a plurality of emitter-coupled differential amplifiers A1, A2, A3, A4 and A5 respectively including amplifying transistor pairs Q1 and Q2, Q4 and Q5, Q7 and Q8, Q10 and Q11, and Q13 and Q14, emitters of each transistor pair being connected to each other.
These differential amplifiers A1, A2, A3, A4 and A5 are enabled or disabled in response to the output signal of a logic controller 5 which controls switching transistors Q3, Q6, Q9, Q12 and Q15 forming a selective switch unit 4. That is, when any one of switching transistors Q3, Q6, Q9, Q12, and Q15 is selected with the output signal of logic controller 5, current from a constant current source I flows into the emitters of the corresponding transistors in the differential amplifier which is enabled due to the switching operation of the selected switching transistor because they are commonly connected to the constant current source I. The first differential amplifier A1 is, for example, driven when switching transistor Q3 of selective switch unit 4 is turned on in accordance with the output signal of logic controller 5, while the remaining switching transistors Q6, Q9, Q12 and Q15 are turned off. In doing so, second through fifth differential amplifiers A2, A3, A4 and A5 are inactive. When driving only switching transistor Q3, amplifying transistors Q1 and Q2 become operative, making signal amplification possible in the first differential transistor A1 when an analog input signal obtained from the first input port IN1 is applied. The amplified analog input signal then appears at output stage 8.
If a second analog input signal is applied to second input port IN2 of input stage 1, and in turn supplied to the base of amplifying transistor Q2 of differential amplifier A1 via bias resistors RB2 and RB1, then the second signal will be output from output stage 8 via common load 3 together with the just analog input signal passing through the first input port IN1, resulting in the generation of crosstalk.
At this point, let us calculate the generated amount of crosstalk from output stage 8. Initially and with reference to FIG. 1 and under a condition that a first switch S1 is shut closed and the other switches, i.e., second through fifth switches S2, S3, S4 and S5 are opened, the generated amount of crosstalk is given below when the analog input signal is applied only to second input port and not to first input port IN1. ##EQU1## where Vct2 is the crosstalk-voltage appearing at output stage 8, Vi2 is an analog input voltage applied to second input port IN2, and Ri1 is an internal resistor included in a first reference voltage source Vref1.
Ideally, the multi-input to single output analog switching device should function so that the undesired analog input signal does not appear at output stage 8. However, a crosstalk phenomenon does occur in the above-mentioned multi-input to single output analog switching device. In the conventional multi-input to single output analog switching device, therefore, such a phenomenon happens in selection of switches other than the first switch S1, for example, while an analog input signal is applied to differential amplifier A1 connected to the first switch S1. To avoid the crosstalk phenomenon, either the internal resistor Ri1 of the first reference voltage source Vref1 must be set in resistance value or substantially attenuated to zero, or the bias voltage applied to respective differential amplifiers must be established from their corresponding independent reference voltage sources. However, this is very difficult to realize in practice.